This invention pertains generally to hybrid coating compositions prepared from silane terminated polyaspartates, polysiloxane resins and polyisocyanates.
The paint industry requires sophisticated coatings, e.g., in the automobile industry, which possess an ideal combination of mechanical properties, such as very high hardness and abrasion resistance, excellent flexibility and very good chemical properties, such as solvent and chemical resistance.
It is known that purely inorganic coating compositions based on siloxanes, which have been produced by reacting hydrolyzable silane groups with water or dehydrating agents, are distinguished by high scratch resistance and chemical resistance (see for example U.S. Pat. No. 4,113,665). A disadvantage of these purely inorganic coatings is that they are brittle and prone to stress cracks. Furthermore, polysiloxane coatings can take hours to days to fully cure.
Hydrolyzable organofunctional silanes are key components for linking conventional organic polymer chemistry with silicone chemistry. Compounds of technical importance for this purpose are in particular those corresponding to the formula (RO)3Si—(CH2)3—Y wherein R is an alkyl group and Y is a functional group. Such compounds contain both hydrolyzable silyl groups, Si—(OR), which crosslink by “silane polycondensation” in the presence of moisture, and other functional groups (Y) which enable them to be chemically linked to conventional polymer materials. (See e.g., Angew. Chem. 98 (1986) 237-253.)
Alkoxysilanes containing amino groups are described, e.g., in J. Org. Chem. 36 (1971), p. 3120; DE-A-1,152,695; DE-A-1,271,712; DE-A-2,161,716; DE-A-2,408,480; DE-A-2,521,399; DE-A-2,749,316; and U.S. Pat. Nos. 2,832,754; 2,971,864; and 4,481,364. Common to all amino-functional silanes known in the art is the disadvantage of being extremely reactive with isocyanates. Therefore, it is difficult to react these alkoxysilanes with polyisocyanates due to the incompatibility, inhomogeneity and extremely high viscosities of the reaction products.
U.S. Pat. Nos. 5,364,955 and 5,766,751 describe silane-terminated resins that have been prepared by reacting NCO prepolymers with silane aspartates to form either urea or hydantoin groups. The silane aspartates are prepared by initially reacting amino-functional silanes with maleic or fumaric acid esters. The silane aspartates are then reacted with NCO prepolymers to form the moisture-curable resins. Condensation products of such aliphatic polyisocyanate-aminosilane adducts with hydrolyzable silanes exhibit an improved balance between high hardness, abrasion resistance and chemical resistance on the one hand and high flexibility on the other. The polyisocyanate-aminosilane adducts react with the hydrolyzable silanes via polycondensation to form flexible inorganic-organic hybrid coatings having high hardness and very good solvent resistance.
An object of the present invention is to improve upon prior art formulations to provide a moisture curable protective coating which does not have to be stoved and is hard cured within minutes of application, yet has a pot life of several hours. An additional object of the present invention is to provide a coating that has good visual quality and exhibits good scratch resistance such that it is suitable for use as an automotive coating, product finish, for industrial and commercial new construction and maintenance, or as OEM coating
These objects may be achieved in accordance with the hybrid coatings of the present invention which are based on silane terminated polyaspartates, polysiloxane resins and polyisocyanates. These coatings hard cure within minutes and provide a chemically resistant, scratch resistant coating having a high gloss.